1. Field of the Invention
This invention relates to a method for testing perpendicular magnetic recording media, and a method for recording servo signals on perpendicular magnetic recording media.
2. Description of the Related Art
In perpendicular magnetic recording, there exist a mode which uses single-layer perpendicular magnetic recording media, and a mode which uses double-layer perpendicular magnetic recording media. In the latter case, there is the problem of spike noise peculiar to double-layer perpendicular magnetic recording media. Perpendicular magnetic recording media have, below the recording layer comprising a perpendicular magnetization film having a high coercive force to retain information, a soft magnetic layer serving as a return path for the recording magnetic field from the recording head during recording. In general, when there exists a soft magnetic film of size equal to that of the magnetic recording media, a plurality of magnetic domains are formed so as to reduce the magnetostatic energy. This is readily understood by practitioners of the art. When such magnetic domains exist, a strong magnetic field emanates from the neighborhood of the magnetic domain walls of these magnetic domains, so that each time a playback head passes over such areas, a spike-shape output is observed. This is what is generally called spike noise. When information or control information used by the magnetic disk device is written at a position where spike noise exists, the waveform of the playback signal is disturbed. When such disturbances occur, the playback of information, or correct operation of the magnetic disk device, is impeded. Spike noise is described for example in the Journal of Applied Physics, Vol. 57, No. 1, pp. 3925-3927 (1985). A description of amplitude modulation of playback signals due to spike noise is given for example in the conference digest of The Fifth Perpendicular Magnetic Recording Conference (PMRC), PMRC 2000, on pages 35 and 36.
Spike noise can be suppressed by, for example, using the technique described in the Journal of the Magnetics Society of Japan, Vol. 21, No. S1, pp. 104-108 (1997). However, even in the case of media in which spike noise is suppressed, prior to use in a product it is necessary that the presence of spike noise be checked, and that, should spike noise exist, the state of distribution thereof be understood. Also, it is of course necessary to understand the distribution of spike noise when developing double-layer perpendicular magnetic recording media.
One of the simplest methods for observing the state of spike noise in double-layer perpendicular magnetic recording media is a method using a spin-stand and oscilloscope. A spin-stand is a device used in magnetic recording experiments, comprising a spindle motor for rotating magnetic recording media, a mechanism to cause a magnetic head to seek a prescribed radial position on the media, an amplifier to operate the head and amplify the output signal thereof, and an amplifier for use in recording. Broadly defined, a spin-stand may further comprise a measurement instruments necessary to perform the magnetic recording experiments, such as to evaluate the basic properties of recording media, as well as a computer to control them. In such cases, the equipment is often called a read/write tester.
When an oscilloscope is used to observe head output, if there exist magnetic domains in the soft magnetic layer of the media, spike noise will of course be observed. The spindle motor normally outputs an index signal (rotation origin signal), and if this is used as a trigger for the oscilloscope, it is possible to determine, for a given radius, the angular position from the rotation origin at which the spike noise appears. Either an analog or a digital storage type oscilloscope may be used. Although this method is suitable for obtaining information relating to the spike noise at a given radial position, it is not suitable for grasping the state of spike noise over the entire media surface. This problem can be resolved by using a digital storage oscilloscope and a computer. As one example, a procedure is described in the IEEE Transactions on Magnetics, Vol. 29, No. 6, pp. 3742-3744 (1993). That is, while varying the radial position at which observations are made, a digital storage oscilloscope is used at each radial position to detect the head output, which is sent to a computer; the amplitude information is converted into a brightness modulation signal, effecting two-dimensional visualization. By using this method, the state of spike noise can easily be grasped intuitively.
The above-described prior art enables either observation of spike noise positions at a given radial position, or two-dimensional visualization of spike noise. These results are qualitative, and so can be used in qualitative evaluations in the research stage; but they cannot be used for pass/fail decisions in production processes. In research and development also, automatic data processing was not possible. Among the above conventional technologies, the latter example has the problem that, due to the large quantity of data obtained, if the results are to be stored without further processing, a large amount of the recording capacity of a storage device would be consumed. Also, if spike noise traverses servo signals, a prominent external disturbance is added to the servo signal read by the head, giving rise to the problem of greatly reduced tracking precision.
On the other hand, while the above-described prior art enabled evaluation of the distribution, amplitude, waveform, and similar of spike noise, when a signal was actually recorded at the position of spike noise, it was not possible to evaluate the effect on the playback signal. Ultimately, the problem for practical purposes might be that signals cannot be recorded correctly, or that recorded signals cannot be played back correctly. In actuality, it has been found that there are at least two types of influence of spike noise on playback signals: baseline shifts, and amplitude modulation. The effects of each of these on the performance of a magnetic recording device differ.